The need for improved pollution control and increased fuel economy have caused internal combustion engine designers to seek substantially improved fuel supply systems. In response, unit fuel injectors having a simplified design have been developed to reduce costs, while providing reliable, precise, and independent control over injector timing and metering. The following patents owned by the assignee of the present application disclose such unit injectors, and are representative of the prior art unit injectors upon which the present invention improves: Perr, U.S. Pat. No. 4,471,909; Peters, U.S. Pat. No. 4,441,654; Peters, U.S. Pat. No. 4,410,138; and Perr, U.S. Pat. No. 4,410,137. All of these patents disclose fuel injectors having an open nozzle and a reciprocating injection plunger mechanically actuated by an engine camshaft.
Despite the advancements achieved heretofore, it had not been possible to obtain sufficiently high injection pressure over the entire range of engine speeds. High pressures (on the order of 30,000 psi and above) are desirable in achieving the higher levels of performance and pollution abatement demanded of modern engines. Additionally, the latter two patents disclose hydraulically controlled injection timing using a timing chamber in which a hydraulic link is formed of a variable length dependent upon the pressure of timing fluid supplied to the injector. When the injector reaches the end of its injection stroke, the timing fluid is dumped through a spillport which is constricted (see Perr '137, col. 12, lines 16-30) to insure sufficiently high pressure in the timing chamber to hold the lower injector plunger in its closed position to resist reopening of the injector spray orifices. While these prior patents disclose important advances, none discloses how to maintain the injector orifice closed near the end of timing fluid chamber collapse when the size of the spillport becomes proportionally too large for the decreasing outflow of timing fluid to maintain adequate pressure within the timing fluid.
New legal restrictions on vehicle emissions have created still higher performance requirements for engine manufacturers which must be met in a cost effective and fuel efficient manner not addressed by the injectors disclosed in the above patents. Dealing with the pollutants at the source, in the combustion chamber, requires increasing the efficiency of the combustion process which requires injecting the fuel at considerably higher pressures, particularly during low speed operation. However, in these injectors the clamped high pressure joints limit the injection pressure capabilities of the fuel injector to sac pressures (fuel pressures in the injection chamber upstream of the injector spray holes) under 20,000 psi. Furthermore, because injection commences shortly after a sealing portion of the plunger has blocked the supply port, the seal length of the plunger presents an interface which leaks if high sac pressure levels (over 30,000 psi) occur.
U.S. Pat. No. 4,721,247 to Perr, also owned by the assignee of the present invention, addresses the problems of achieving high sac pressures throughout the entire range of engine speeds. Perr '247 discloses an open nozzle type unit fuel injector capable of achieving sac pressures exceeding 30,000 psi during injection even at low engine speed. This type of injector is known as a high pressure injector (HPI) and includes a plunger assembly having three plungers arranged to form a hydraulic variable timing fluid chamber between the upper and intermediate plungers and an injection chamber below the lower plunger. An increase in sac pressure is obtained under both low and high speed operating conditions by being designed to achieve high pressures at low engine speed and by being provided with a pressure actuated valve for draining timing fluid from the timing chamber when the engine is operated at higher speed.
The '247 patent uses a single spring mounted between the intermediate and lower plunger to bias the intermediate plunger upwardly. By careful design of the spring rate characteristics of the intermediate plunger bias spring, it becomes possible to control the amount of timing fluid which is metered into the timing chamber during each cycle of injector operation by changing the pressure of the timing fluid supplied to the injector. However, in the '247 patent, the intermediate plunger bias spring also supplies the bias force necessary to operate the pressure actuated relief valve. Accordingly, it becomes very difficult to optimize timing fluid metering without affecting adversely the operation of the pressure actuated relief valve and vice versa. Moreover, the size of the drain passage from the timing chamber affects both the opening pressure of the pressure limiting valve and the flow rate of timing fluid drained from the timing chamber, through the pressure limiting valve. Thus, although fuel injectors having relatively simple designs capable of high injection pressure at low operating speed conditions have been developed, there is still a need for an injector that allows independent control over the metering of timing fluid and the opening characteristics of the timing pressure limiting valve, and separate control of the opening pressure of the pressure limiting valve and the flow rate of timing fluid discharge flow.
No known prior art fuel injector incorporates a system for reducing wear, increasing durability, and increasing performance characteristics by means of a variable length hydraulic link forming timing chamber and associated structure whereby the timing fluid metering function can be optimized independently of the pressure limiting valve mechanism which allows high pressure operation of the injector at low engine speed. Similarly, the prior art fails to disclose an end-of-injection spillport mechanism for maintaining the injector orifices closed to eliminate secondary injection.